BIAXIAL EXTENSION OF WHEAT FLOUR DOUGHS: LUBRICATED SQUEEZING FLOW AND STRESS RELAXATION PROPERTIES

Abstract

ABSTRACT Biaxial extension in lubricated squeezing flow (LSF) was first developed for molten polymers and transferred to the study of wheat flour dough (1986). Depending on geometry, LSF is performed at constant surface or at constant volume. Most of the measurements have been carried out with a mechanical testing machine at constant crosshead speed and constant surface. An empirical power law equation relates stress and strain rate at constant strain and, at a given strain rate, there is an approximate exponential stress–strain relationship from which a parameter named “strain hardening” is derived and tentatively related to resistance of dough membranes against premature rupture. This exponential relationship does not seem to hold in other experimental conditions, particularly at constant volume. Stress relaxation following lubricated compression might also allow the study of dough viscoelastic properties. The role of unsteady regime, testing conditions and dough compressibility is finally discussed. PRACTICAL APPLICATIONSExtensional flow properties of polymers have long been recognized as important for understanding their processing behaviors and the experimental measurement of these properties has been the subject of many studies. The relevance of biaxial extension properties to dough behavior in baking technology was at the origin of the Alveograph developed by Chopin more then 80 years ago. A third method, lubricated squeezing flow (LSF), was firstdeveloped in 1981 for studying the biaxial extension viscosity of molten polymers in creep. It is based on the uniaxial compression of a cylindrical sample between two parallel surfaces. Due to its simplicity, versatility and availability, LSF has found several empirical applications in food, particularly in wheat flour dough rheology.